1. Field of the Invention
The present invention relates to a nitride-based semiconductor light-emitting device, and more specifically, it relates to a nitride-based semiconductor light-emitting device including a nitride-based semiconductor layer represented by a group III nitride-based semiconductor.
2. Description of the Background Art
An ultraviolet LED, a blue LED or a green LED employing a nitride-based semiconductor consisting of InXAlYGa1-YN (0xe2x89xa6X, 0xe2x89xa6Y, X+Yxe2x89xa61) is recently put into practice. Such an LED basically has a double hetero structure obtained by successively stacking an n-type nitride-based semiconductor layer consisting of n-type AlYGa1-YN (0xe2x89xa6Yxe2x89xa61), an emission layer consisting of InXGa1-XN (0xe2x89xa6Xxe2x89xa61) and a p-type nitride-based semiconductor layer consisting of p-type AlYGa1-XN (0xe2x89xa6Zxe2x89xa61), for example, on a transparent insulating substrate.
In relation to the nitride-based semiconductor light-emitting device having the aforementioned double hetero structure, a structure obtained by providing a light-transmitting p-side electrode consisting of a metal on a p-type nitride-based semiconductor layer defining an emission observation surface for externally extracting emission from the emission layer is known in general. This structure is disclosed in Japanese Patent Laying-Open No. 6-314822 (1994), for example.
A p-side electrode and an n-side electrode employed for an LED having the aforementioned structure must be in excellent ohmic contact with a p-type nitride-based semiconductor layer and an n-type nitride-based semiconductor layer in contact with the p-side electrode and the n-side electrode respectively, in order to reduce a forward voltage. In general, therefore, the n-side electrode contains Ti and Al exhibiting excellent ohmic contact with the n-type nitride-based semiconductor layer. Further, the p-side electrode having light transmittance contains Ni and Au exhibiting excellent ohmic contact with the p-type nitride-based semiconductor layer.
When forming the p-side electrode having light transmittance on the aforementioned conventional p-type nitride-based semiconductor layer, the thickness of the p-side electrode must be increased for reducing electric resistance, in order to reduce the forward voltage of the LED. When the thickness of the p-side electrode is increased as described above, however, the transmittance of the p-side electrode is disadvantageously reduced with respect to blue light and green light. Thus, luminous intensity of light extracted from the p-side electrode is disadvantageously reduced.
When the thickness of the p-side electrode is reduced in order to improve the transmittance of the p-side electrode, on the other hand, the sheet resistance of the p-side electrode is increased to increase the contact resistance between the p-side electrode and the p-type nitride-based semiconductor layer. Therefore, a current hardly homogeneously flows from the p-side electrode to the p-type nitride-based semiconductor layer, and hence it is difficult to attain homogeneous emission. The luminous intensity is disadvantageously reduced also in this case. In order to prevent such reduction of the luminous intensity, the driving voltage for the LED must disadvantageously be increased.
An object of the present invention is to provide a nitride-based semiconductor light-emitting device capable of attaining homogeneous emission with a low driving voltage.
Another object of the present invention is to reduce the contact resistance between an electrode and a nitride-based semiconductor layer (contact layer) in the aforementioned nitride-based semiconductor light-emitting device.
Still another object of the present invention is to further reduce the sheet resistance of a light-transmitting electrode in the aforementioned nitride-based semiconductor light-emitting device.
A further object of the present invention is to reduce height of discontinuity of band gaps of the contact layer and a cladding layer in the aforementioned nitride-based semiconductor light-emitting device.
A nitride-based semiconductor light-emitting device according to an aspect of the present invention comprises a first conductivity type first nitride-based semiconductor layer formed on a substrate, an emission layer, consisting of a nitride-based semiconductor, formed on the first nitride-based semiconductor layer, a second conductivity type second nitride-based semiconductor layer formed on the emission layer, a second conductivity type intermediate layer, consisting of a nitride-based semiconductor, formed on the second nitride-based semiconductor layer, a second conductivity type contact layer, including a nitride-based semiconductor layer having a smaller band gap than gallium nitride, formed on the intermediate layer, and a light-transmitting electrode formed on the contact layer.
The nitride-based semiconductor light-emitting device according to the first aspect is provided with the second conductivity type contact layer including the nitride-based semiconductor layer having a smaller band gap than gallium nitride as hereinabove described, whereby the contact layer including the nitride-based semiconductor layer having a smaller band gap than gallium nitride can attain a higher carrier concentration than a contact layer (nitride-based semiconductor layer) consisting of gallium nitride and hence the thickness of a barrier formed on the interface between the contact layer and the electrode can be reduced. Thus, the contact resistance between the contact layer and the light-transmitting electrode can be reduced. Consequently, homogeneous emission can be attained while a driving voltage can be reduced. The nitride-based semiconductor layer having a smaller band gap than gallium nitride has higher electric conductivity than gallium nitride, and hence a current readily homogeneously spreads in the nitride-based semiconductor layer having a smaller band gap than gallium nitride. Thus, homogeneous emission can be attained also when the light-transmitting electrode is formed in a small thickness. When the second conductivity type intermediate layer is so formed as to substantially have an intermediate band gap between those of the second conductivity type contact layer and the second conductivity type second nitride-based semiconductor layer (cladding layer), the intermediate layer can reduce height of discontinuity of the band gaps of the contact layer and the cladding layer. Thus, resistance against a current flowing from the contact layer to the cladding layer can be reduced, thereby obtaining a nitride-based semiconductor light-emitting device having high luminous efficiency.
In the nitride-based semiconductor light-emitting device according to the aforementioned aspect, the carrier concentration of the second conductivity type contact layer is preferably at least 5xc3x971018 cmxe2x88x923. According to this structure, the thickness of the barrier formed on the interface between the contact layer and the electrode can be so reduced that the contact resistance between the contact layer and the light-transmitting electrode can be readily reduced. Consequently, homogeneous emission can be attained and the driving voltage can be reduced.
In the aforementioned case, the second conductivity type contact layer preferably contains gallium indium nitride. According to this structure, the band gap of the contact layer can be readily reduced below that of gallium nitride.
In the aforementioned case, the light-transmitting electrode preferably contains at least one material selected from a group consisting of nickel, palladium, platinum and gold. According to this structure, excellent ohmic contact can be attained between the light-transmitting electrode and the contact layer.
In the aforementioned case, the light-transmitting electrode is preferably formed in a thickness capable of transmitting light. According to this structure, the electrode can be readily provided with light transmittance.
In the aforementioned case, the light-transmitting electrode preferably has a thickness of not more than 10 nm. According to this structure, the electrode can be provided with excellent light transmittance.
In the aforementioned case, the light-transmitting electrode is preferably formed to have a window capable of transmitting light. According to this structure, the thickness of a portion other than the window can be so increased as to further reduce the sheet resistance of the light-transmitting electrode. In this case, a region of the light-transmitting electrode defining the window may be formed in a thickness capable of transmitting light, and the remaining region of the light-transmitting electrode other than the window may be formed in a thickness larger than the thickness capable of transmitting light. Further, the light-transmitting electrode may not be formed in a region defining the window capable of transmitting light, and the remaining region of the light-transmitting electrode other than the window may be formed in a thickness larger than the thickness capable of transmitting light. According to this structure, the sheet resistance of the light-transmitting electrode can be further reduced.
In this case, the electrode having the window capable of transmitting light may include a mesh electrode. Alternatively, the electrode having the window capable of transmitting light may include a comb-shaped electrode. Further alternatively, the electrode having the window capable of transmitting light may include a meander electrode. According to this structure, the light transmittance can be improved due to the window, while the sheet resistance of the light-transmitting electrode can be further reduced by increasing the thickness of the portion other than the window.
In the aforementioned case, the first conductivity type first nitride-based semiconductor layer preferably contains gallium nitride. Further, the emission layer preferably contains gallium indium nitride.
In the aforementioned case, the first nitride-based semiconductor layer is preferably formed on the substrate through a buffer layer. According to this structure, the density of dislocations in the first nitride-based semiconductor layer can be reduced also when the substrate is different in lattice constant from the nitride-based semiconductor. In this case, the nitride-based semiconductor light-emitting device preferably further comprises a low-dislocation-density second nitride-based semiconductor layer formed on the buffer layer by lateral growth, and the first nitride-based semiconductor layer is preferably formed on the second nitride-based semiconductor layer. According to this structure, the density of dislocations in the first nitride-based semiconductor layer can be further reduced, whereby the luminous efficiency can be further improved.
In the aforementioned case, the substrate preferably includes a substrate selected from a group consisting of a sapphire substrate, a spinel substrate, an Si substrate, an SiC substrate, a GaAs substrate, a GaP substrate, an InP substrate, a quartz substrate, a ZrB2 substrate and a GaN substrate.
In the aforementioned case, the second conductivity type intermediate layer preferably substantially has an intermediate band gap between the band gap of the second conductivity type contact layer and the band gap of the second conductivity type second nitride-based semiconductor layer. According to this structure, the intermediate layer can relax discontinuity of the band gaps of the contact layer and the second nitride-based semiconductor layer, whereby resistance against the current flowing from the contact layer to the second nitride-based semiconductor layer can be reduced. Consequently, the luminous efficiency can be improved.
In this case, the second conductivity type second nitride-based semiconductor layer preferably contains a nitride-based semiconductor having a larger band gap than gallium nitride. According to this structure, height of discontinuity of the band gaps of the contact layer and the second nitride-based semiconductor layer can be readily redused when the intermediate layer is made of gallium nitride, for example.
In this case, the second conductivity type second nitride-based semiconductor layer preferably contains gallium aluminum nitride. According to this structure, the surface of the second nitride-based semiconductor layer can be prevented from degradation in growth by forming the intermediate layer consisting of a material having a smaller aluminum composition as compared with the aforementioned second nitride-based semiconductor layer containing gallium aluminum nitride or containing no aluminum. Thus, the upper surface of the second nitride-based semiconductor layer is prevented from formation of a degraded layer of high resistance, whereby the resistance against the current flowing from the contact layer to the second nitride-based semiconductor layer can be further reduced. Further, inequality of in-plane resistance resulting from degradation of the second nitride-based semiconductor layer can be reduced, thereby reducing irregular emission. Thus, homogeneous emission can be attained.
In this case, the second conductivity type intermediate layer preferably contains either gallium nitride or gallium indium nitride. According to this structure, gallium nitride or gallium indium nitride containing no aluminum can readily prevent the surface of the second nitride-based semiconductor layer from degradation.
In this case, the second conductivity type intermediate layer may have a composition continuously changing from the second conductivity type second nitride-based semiconductor layer toward the second conductivity type contact layer. According to this structure, height of discontinuity of the band gaps of the contact layer and the second nitride-based semiconductor layer can be further reduced.
The nitride-based semiconductor light-emitting device according to the aforementioned aspect may include a light-emitting diode device.
In the nitride-based semiconductor light-emitting device according to the aforementioned aspect, the second conductivity type contact layer may be doped with a p-type impurity by at least 5xc3x971018 cmxe2x88x923. According to this structure, the thickness of the barrier formed on the interface between the contact layer and the electrode can be so reduced that the contact resistance between the contact layer and the light-transmitting electrode can be readily reduced. Consequently, homogeneous emission can be attained and the driving voltage can be reduced.
In the nitride-based semiconductor light-emitting device according to the aforementioned aspect, the second conductivity type contact layer may be subjected to modulation doping to exhibit the second conductivity type. According to this structure, the thickness of the barrier formed on the interface between the contact layer and the electrode can be so reduced that the contact resistance between the contact layer and the light-transmitting electrode can be readily reduced. Consequently, homogeneous emission can be attained and the driving voltage can be reduced.
In the nitride-based semiconductor light-emitting device according to the aforementioned aspect, the second conductivity type may be the p-type.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.